Muscular System
1. State the characteristics and functions of muscle tissue.
2. Describe the structure of a skeletal muscle.
3. List and describe the sequence of events involved in the contraction of a skeletal muscle fiber.
4. Explain how energy is provided for a muscle contraction.
6. Describe and illustrate the movements accomplished by the contraction of skeletal muscle.
7. Identify and describe the major muscles making up the axial skeleton.
8. Identify and describe the major muscles making up the appendicular skeleton.
9. Describe ways in which the aging of an individual affects the muscular system.
Introduction to the Muscular System
As described in Chapter 5, there are three types of muscle tissue: skeletal, visceral, and cardiac. These are reviewed in Table 8-1. This chapter takes a closer look at skeletal muscle, which makes up about 40% of an individual’s body weight. It forms more than 600 muscles that are attached to the bones of the skeleton. Skeletal muscles are under conscious control, and when they contract they move the bones. Skeletal muscles also allow us to smile, frown, pout, show surprise, and exhibit other forms of facial expression.
Characteristics and Functions of the Muscular System
Skeletal muscle has four primary characteristics that relate to its functions:
Excitability: Excitability (eks-eye-tah-BILL-ih-tee) is the ability to receive and respond to a stimulus. To function properly, muscles have to respond to a stimulus from the nervous system.
Contractility: Contractility (kon-track-TILL-ih-tee) is the ability to shorten or contract. When a muscle responds to a stimulus, it shortens to produce movement.
Extensibility: Extensibility (eks-ten-sih-BILL-ih-tee) means that a muscle can be stretched or extended. Skeletal muscles are often arranged in opposing pairs. When one muscle contracts, the other muscle is relaxed and stretched.
Elasticity: Elasticity (ee-lass-TISS-ih-tee) is the capacity to recoil or return to the original shape and length after contraction or extension.
Muscle contraction fulfills four important functions in the body:
Nearly all movement in the body is the result of muscle contraction. Some exceptions to this are the action of cilia, the motility of the flagella on sperm cells, and the ameboid movement of some white blood cells. The integrated action of joints, bones, and skeletal muscles produces obvious movements such as walking and running. Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration. Posture, such as sitting and standing, is maintained as a result of muscle contraction. The skeletal muscles are continually making fine adjustments that hold the body in stationary positions. Skeletal muscles contribute to joint stability. The tendons of many muscles extend over joints and in this way contribute to joint stability. This is particularly evident in the knee and shoulder joints, where muscle tendons are a major factor in stabilizing the joint. Heat production, to maintain body temperature, is an important by-product of muscle metabolism. Nearly 85% of the heat produced in the body is the result of muscle contraction.
Structure of Skeletal Muscle
A whole skeletal muscle is considered an organ of the muscular system. For example, the biceps muscle is an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.
Whole Skeletal Muscle
An individual skeletal muscle such as the biceps muscle may consist of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium (ep-ih-MYE-see-um). Fascia consists of connective tissue located outside the epimysium. Fascia surrounds and separates the muscles. Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The connective tissue coverings furnish support and protection for the delicate cells and allow them to withstand the forces of contraction. The coverings also provide pathways for the passage of blood vessels and nerves.
Highlight on the Muscular System
Rigor mortis: The term rigor mortis means the “stiffness of death.” Within a short time of death, the adenosine triphosphate in muscles breaks down. This causes the myofilaments to remain locked in a contracted position and the body becomes rigid. A day or so later, muscle proteins begin to deteriorate and the rigor mortis disappears.
Tetanus: The word tetanus is often confusing because it means different things to different people. In reference to muscle contraction, the term denotes a steady contraction of a muscle fiber, without a relaxation phase. The word also refers to a disease, commonly called “lockjaw,” that is caused by the bacterium Clostridium tetani. The toxin from the bacteria causes nerves to be highly excitable, which, in turn, causes uncontrollable muscle contractions, or spasms. A third use of the word is to denote a condition caused by a deficiency of calcium ions in the extracellular fluid. The lack of calcium increases nerve excitability with resulting muscle spasms, particularly of the extremities. The word tetany is also sometimes used to mean tetanus.
Cramps: Cramps are painful, spastic contractions of muscles. They are usually caused by an irritation within the muscles that results in reflex contractions. Local inflammation from the accumulation of lactic acid is one source of irritation.
Wryneck: Injury to one of the sternocleidomastoid muscles may result in torticollis, or wryneck. This is characterized by a twisting of the neck and an unnatural position of the head.
Diaphragm: Voluntary forceful contractions of the diaphragm increase intraabdominal pressure to assist in urination, defecation, and childbirth.
Electrical shock: The muscles that flex the fingers and hand are stronger than the extensor muscles. In a normal relaxed position the fingers are slightly flexed because the normal muscle tone is greater in the flexors. Persons who receive a high-voltage electrical shock through the arms flex their hands tightly and “can’t let go.” All of the flexors and extensors receive the electrical stimulus, but because the flexor muscles are stronger, they contract more forcefully.
Intramuscular injections: The gluteus medius is a common site for intramuscular injections. Generally, the injection is given in the center of the upper outer quadrant of the buttock, or gluteal, area. The gluteus medius, rather than the gluteus maximus, is used to avoid damaging the sciatic nerve.
Horseback riding: The adductor muscles in the medial compartment are the horse rider’s muscles. These muscles adduct, or press, the thighs together to keep a person on a horse.
Quads: The quadriceps femoris group is a powerful knee extensor that is used in climbing, running, and rising from a chair. 
Skeletal Muscle Fibers
Each individual skeletal muscle fiber consists of a single cylindric muscle cell. The cell membrane is called the sarcolemma (sar-koh-LEM-mah), and the cytoplasm is the sarcoplasm (SAR-koh-plazm). Multiple nuclei are next to the sarcolemma at the periphery of the cell. Because the muscle cell needs energy for contraction, there are numerous mitochondria.
Nerve and Blood Supply
Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle contraction. Before a skeletal muscle fiber can contract, it must receive an impulse from a nerve cell. Muscle contraction requires adenosine triphosphate (ATP), and blood vessels deliver the necessary nutrients and oxygen to produce it. Blood vessels also remove the waste products that are produced as a result of muscle contraction.
In general, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle so that each muscle fiber is in contact with a branch of a nerve cell and with one or more minute blood vessels called capillaries.
Skeletal Muscle Attachments
In some instances, fibers of the epimysium fuse directly with the periosteum of a bone to form a direct attachment. The fleshy part of the muscle is known as the belly or gaster. More commonly, the connective tissue coverings extend beyond the belly of the muscle to form a thick, ropelike tendon or a broad, flat, sheetlike aponeurosis (ah-pah-noo-ROE-sis). The tendons or aponeuroses form indirect attachments from muscles to the periosteum of bones or to the connective tissue of other muscles. Typically, a muscle spans a joint and is attached to bones by tendons at both ends. One of the bones remains relatively fixed or stable while the other end moves as a result of muscle contraction. The fixed or stable end is called the origin of the muscle, and the more movable attachment is called the insertion.
Contraction of Skeletal Muscle
Skeletal muscle contraction is the result of a complex series of events based on chemical reactions at the cellular (muscle fiber) level. This chain of reactions begins with stimulation by a nerve cell and ends when the muscle fiber is again relaxed. Contraction of a whole muscle is the result of the simultaneous contraction of many muscle fibers.
Stimulus for Contraction
Skeletal muscles are stimulated to contract by special nerve cells called motor neurons. As the axon of the motor neuron penetrates the muscle, the axon branches, so there is an axon terminal for each muscle fiber. A single motor neuron and all the muscle fibers it stimulates make up a motor unit. Some motor units include several hundred individual fibers; others contain fewer than 10. Because all the muscle fibers in a motor unit receive a nerve impulse at the same time, all the fibers contract at the same time.
The region in which an axon terminal meets a muscle fiber is called a neuromuscular junction or myoneural junction, which is illustrated in Figure 8-1. The axon terminal does not actually touch the sarcolemma of the muscle cell but fits into a shallow depression in the cell membrane. The fluid-filled space between the axon terminal and sarcolemma is called a synaptic cleft (gap). Acetylcholine (ACh) (ah-see-till-KOH-leen), a neurotransmitter, is contained within synaptic vesicles in the axon terminal. Receptor sites for the ACh are located on the sarcolemma.
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